1. Field of the Invention
Embodiments of the present invention generally relate to electron guns (sources), and more particularly, electron guns that may be used, for instance, in electron beam lithography or electron microscopy.
2. Description of the Related Art
Electron beam columns are well known for use, for instance, in electron beam lithography for imaging a pattern onto a substrate typically coated with a resist sensitive to electron beams. Subsequent development of the exposed resist defines a pattern in the resist which later can be used as a pattern for etching or other processes. Electron beam columns are also used in electron microscopy for imaging surfaces and thin samples. Conventional electron beam columns for electron microscopy and lithography are well known and typically include an electron gun having an electron emitter for producing an electron beam. The beam from the gun may be used to produce a scanning probe or illuminate a sample or an aperture using a series of electron beam lenses, which may be magnetic or electrostatic.
Electron beam columns generally include a source of electrons, such as a Schottky emission gun or a field emission gun, which typically includes an emitter (cathode), an electrostatic pre-accelerator lens that focuses the electron beam and a series of lenses that refocuses and images the source aperture or sample onto the target.
It has generally been difficult to obtain very high beam currents focused into a small spot using Schottky electron sources. Although the brightness of the emitter is high in such sources, the angular intensity of the electron beam emerging from the emitter region is limited by the properties of the emitter itself. Consequently, a rather large aperture angle must be used in the electron gun, which makes spherical and chromatic aberration in the gun lens a major factor in limiting the small spot size that can be achieved, which is generally referred to as the smallest cross-section diameter of the beam.
One approach to reduce aberrations in the gun lens is to use a magnetic lens as the focus element. Using this approach, the emitter tip and the extraction region are immersed in a magnetic field, which results in a significant increase in the operating solid angle of emission compared to all-electrostatic systems. However, one disadvantage of this design is that the lens coil and its cooling fluid may float at near the tip potential, which requires a more complicated high voltage power supply and cable. Further, the mechanical design is a large departure from conventional Schottky or field emission designs, which adds further complication to the approach.
Other attempts to reduce aberrations in the gun lens have been made. However, those attempts have proven to be difficult since the size and focal length of standard electrostatic lenses are limited by the large stand-off distance required in high voltage systems.
Therefore, a need exists in the art for a new electron beam source for an electron gun with minimal aberrations.